The present invention relates to a process for at least partially coating workpieces using a sputter-CVD process, and to the workpiece which is coated by the process.
A process of this kind has been described by C. E. Wickersham, E. L. Foster and G. H. Stickford in "Reactively Sputter-Deposited High-Emissivity Tungsten Carbide/Carbon Coatings", J. Vac. Sci., Technol., 18 (2), March 1981, p. 223 to 225. There a workpiece was coated with a tungsten carbide/carbon layer. The cathode sputtering operated with a frequency of 13.56 MHz at a power of 500 W and a negative bias on the target. Acetylene is used as the reactive gas. This is a type of chemical vapor deposition or CVD process.
The process of the above-mentioned kind (sputter-CVD) must be distinguished firstly from other plasma-assisted chemical vapor on (CVD) processes, in which the chemical reactants of the substance forming the layer are introduced into the vacuum chamber in gaseous form and are activated and partially ionized in the space before the workpiece surfaces to be coated by an electric gas discharge (low-voltage arc) (Swiss Pat. No. CH-PS 664,768), the workpieces being connected to a voltage which is negative as compared with the vacuum chamber and the arc plasma In contrast to the initially mentioned process, cathode sputtering does not take place here.
Secondly, the sputter-CVD process of the kind in question must be distinguished from Physical Vapor Deposition (PVD), where the substances are applied to the workpiece surface without chemical reaction, and only by condensation of vapor. The vapor can be produced by cathode sputtering, see for instance J. L. Vossen, J. J. Cuomo, in "Thin Film Processes", Academic Press, 1978, p. 56 and 57. As described there, one operates either with DC voltage or a high-frequency glow discharge, and the workpieces are connected to a negative bias voltage. A positive bias should be avoided as Vossen et al. state, for several reasons, mainly for the reason that a strong electron flow onto the workpieces occurs, with the result of intense substrate heating and very irregular current distribution.
The known sputter-CVD process thus is indeed advantageous as compared with the other mentioned processes, with respect to the layer composition and layer properties. But the excitation of the gas by a high-frequency voltage results in high machine investments as compared with a system operating with DC voltage because of the necessary high-power HF generators, which must be shielded in accordance with legal regulations. Further, it is difficult to introduce the high frequency into the vacuum coating chamber without great losses, especially if the parts to be coated are held by rotating supports during the coating operation.